160 GENERAL ZOOLOGY 



sponges are related, and that the Porifera arose independ- 

 ently from the Protozoa. The Metazoa are, therefore, 

 divided into two groups (page 24) : the Enterozoa, in- 

 cluding most of the phyla; and the Parazoa, containing only 

 the sponges. 



These theoretical matters regarding classification are not 

 to the point, however, and we will return to the develop- 

 ment of the sponge. A ciliated larva is formed within a 

 special cavity, or brood pouch, after cleavage inside the 

 body of its parent. The larva when set free leaves its 

 parent's body in the water currents and swims about 

 actively until it finds some suitable place for attachment; 

 then settles down, loses its ciliated covering, develops pores 

 through the tissues, and grows into a mature sponge. 



In its mature state a sponge is barely across the line be- 

 tween the Protozoa and the Metazoa. It has tissues, but 

 that is about all that can be granted it ! There is little or 

 no correlation between the different parts of the body, and 

 there are no organs. Each tissue does its work more or less 

 automatically and is rather independent of others. The 

 flagellated choanocytes beat continually until they die, and 

 will always make currents if the canals are open. If some 

 injurious substance is entering with the water, it can be 

 excluded only by closing the openings, and such closure is 

 brought about by the direct action of the substance on the 

 little sphincters, or rings of muscular tissue, surrounding 

 the openings. If a certain part of a sponge's body is cut, it 

 may contract and adjacent pore openings may close, but 

 there is no transmission of the disturbance to other parts of 

 the body. The sponge is, therefore, a rather poorly organ- 

 ized community of tissues. Muscular and supporting 

 tissues are present but there are no nerves, nor are there 

 evidences of nervous transmission. This .is an interesting 

 fact from a psychological point of view for it indicates that 

 the nervous system in animals generally has arisen after 

 various effectors (muscles, glands, etc.), and has secondarily 

 come to control them. A sponge has effectors (parts of the 



